180 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



238. Warmth required for Photosynthesis. It is easy to 

 study the influence of temperature upon photosynthesis by 



noting the results of warming or cooling the air in contact with the 

 leaves of land-plants, or the water in which aquatic plants are 

 placed, setting up in each case a control experiment in which the 

 plants are subjected to the ordinary temperature. 



(1) Place a starch-free leaf (or a whole plant with starch-free 

 leaves) of Tropaeolum, Fuchsia, or Primrose, in a jar kept cold by 

 ice, expose to light, and after an hour or two test with iodine. Set 

 up a control experiment along with this, with everything the same 

 except that no ice is used. 



(2) Time the rate of bubbling of oxygen from a shoot of Elodea 

 or other submerged aquatic. Drop pieces of ice into the water, 

 read the temperature, and note that the bubbling becomes slower. 



239. The Influence of Light Intensity on Photo- 

 synthesis may be demonstrated, roughly at any rate, by 

 such experiments as the following : 



(1) Cover half of a starch-freed leaf with a piece of 

 fairly thin white paper, or a piece of ground glass, and 

 expose to light ; after a few hours, remove the paper or 

 glass and decolorise and test the leaf with iodine. 



(2) Instead of using a tinfoil stencil, cover the upper 

 side of a starch-freed leaf with a photographic negative ; 

 or fit a 5 x 4 negative into the light- screen shown in 

 Fig. 42. After exposure to light, test with iodine; a 

 " starch print " is obtained, in which the lightest parts of 

 the negative show up darkest in the " starch print," and 

 vice versa. 



(3) Place some healthy cut branches of Elodea or other 

 water-plant under water, and select one which gives a good 

 stream of oxygen-bubbles (fairly rapid and constant) from 

 its cut end. Count the time required for, say, ten bubbles 

 to be given off, and repeat the counting several times till 

 you get a fairly constant result. Then remove the jar into 

 the shade, or cover it with a sheet of thin white paper to 

 weaken the light, and take times as before, noting the 

 change in the rate of bubbling. 



(4) Set the jar containing the water-plant under a box 

 open at one side, and throw light on the plant from this 

 side by placing a lamp at different distances from it, noting 

 the distances and the rates of bubbling. Bring the lamp 



